628 Obstructions in Draining-Tiles. 



turbid in a few moments, and begins to deposit the ocbreous matter 

 which forms the basis of tlie obstructions of which we are treating^. 

 The deposit collected in the drains, or in the ditches into which 

 they discharge themselves, may be easily freed from the liquid 

 by washings with pure water. By exposure to air the tint 

 becomes more red. When after some hours it appears no longer 

 to vary in colour, the deposit is placed in a botde tilled with 

 water and well corked : the red tint will be seen to become by 

 degrees dark brown or almost black. After some weeks, if 

 the produce is filtered, a pure liquid is again obtained, but 

 which rapidly becomes turbid by exposure to air, and allows 

 the ochreous deposit, of which I have spoken, to form. At the 

 same time the deposit left behind in the filter resumes the red 

 tint which it presented at the moment it was enclosed in the 

 bottle. The same series of observations may be repeated several 

 times by the same sample. The product in question presents 

 then this double character : it becomes insoluble by its oxyda- 

 tion, and it is able, when left to itself, to reduce itself, so as to 

 become partly soluble. If you place three or four cubic centi- 

 metres of the ochreous precipitate, recently collected, and saturated 

 with the water from out of which it was formed, in a prover filled 

 with oxygen, secured over a bowl of mercury, the absorption of 

 gas is at first very rapid, then it slackens by degrees, and at last 

 ceases altogether. During the first eight days of one of my ex- 

 periments 14 cubic centimetres of gas were absorbed, Avhile on 

 the thirteen following days 5 cubic centimetres only disappeared. 

 The mass was then completely of a red tint, and, put through a 

 filter, gave a clear liquid, and did not contain in solution any 

 product worthy of remark. The liquid which impregnates the 

 new precipitates contains variable proportions of substances 

 precipitaljle by the action of air. We have obtained up to 

 0*80 per litre, although the action of the oxygen had already pre- 

 cipitated a part of it. Commonly from 025 to 0"50 per litre will- 

 be found, which is sufficient, on account of the lightness of the 

 product and its gelatinous consistency, quickly to produce an 

 obstruction in tiles. From these facts it results — 



1. That the waters which produce the ferruginous obstruc- 

 tions in tiles preserve their limpidity, and do not form any 

 deposit, when they are placed beyond the reach of the oxygen of 

 the air. 



2. That a deposit recently formed can exert upon itself a 

 reducing action which makes it in a great degree return to 

 a soluble state. 



From these two facts it is easy to conclude that air-traps 

 resembling those described in speaking of calcareous deposits, 

 will equally prevent the formation of ochreous deposits in drain- 

 tiles. In the second case the trap, instead of preventing the dis- 



